1. Field of the Invention
In general, the present invention relates to a plasma processing method and a plasma processing apparatus. More particularly, the present invention relates to a plasma processing method and a plasma processing apparatus that are suitable for a process to etch a substrate such as a semiconductor wafer by using plasma.
2. Related Background Art
A technology known as a technology for sustaining etching performance is disclosed in Japanese Patent Laid-Open No. H9-129594. As disclosed in the document, this technology provides a capability of control resulting in high etching uniformity, excellent pattern dimensions and a superior pattern cross-sectional shape by controlling a bias voltage in accordance with variations in plasma state detected by adoption of at least one of methods listed below after generating plasma in a gas including a reaction gas by application of a power to a first electrode. The methods are a plasma emission analysis, a mass analysis of a substance in the plasma, a measurement of a self-bias voltage of the plasma and a measurement of an impedance value of the plasma.
In Japanese Patent Laid-Open No. H11-297679, there is disclosed a technology known as a technology for fabricating a device with a fabricated-line dimension up to 1 micron to keep up with miniaturization of semiconductor devices. As disclosed in the document, this technology provides a method for fabricating a surface of a sample whereby the sample is placed on a sample base provided in a vacuum container, a processing gas is supplied to the inside of the container to be converted into plasma, a high-frequency bias with a frequency of at least 100 kHz is applied to the sample base independently of the generation of the plasma, the high-frequency bias is modulated by a frequency in the range 100 Hz to 10 kHz and the voltage of the high-frequency bias is subjected to on-off control of its peak-to-peak voltage Vpp. This peak-to-peak voltage Vpp is greater than the peak-to-peak voltage Vpp of a continuous high-frequency bias required to generate the same etching speed as the on-off control.
With semiconductor devices"" speed enhancement in recent years, at the present time, the fabricated-line dimension of LSIs (Large Scale Integrated Circuits) has reached a level of 0.1 microns. Thus, it is necessary to provide a fabrication precision of xc2x10.01 microns for a device""s electrodes and wires.
With an etching apparatus using plasma, on the other hand, there is raised a problem that the fabricated-line dimension slightly varies from wafer to wafer. For example, in an etching apparatus, the plasma is affected by the shape of the inner wall of a vacuum container and other causes. That is, when an Si wafer is etched, a substance obtained as a result of a reaction of Si is stuck to the inner wall and changes the state of the surface of the inner wall. In addition, the stuck substance is later released from the surface of the inner wall. These processes of sticking of such a substance to the inner wall as well as releasing the substance from the wall and other processes change the composition of the plasma. As a result, in sequential wafer processing to process a wafer after another, the fabricated-line width slightly varies from wafer to wafer even if the conditions of the wafer processing such as the gas"" flow and pressure are maintained all the time. In the case of a fabricated-line dimension at the 0.1-micron level accompanying device miniaturization in recent years, these dimension variations, which do not raise a problem in the fabricated-line dimension at the 0.5-micron level, cause a problem of a difficulty to satisfy required fabrication precision.
In order to solve this problem, there is provided a method referred as in-situ cleaning. That is, in accordance with this method, a chamber is cleaned after each wafer processing. However, this method causes the throughput to decrease and cannot be said to be effective for all plasma processes. There is also provided another conceivable method whereby processing conditions are changed for each wafer or for each plurality of wafers. As such a method, there is provided a feedback control method like the one described in the Related Background Art.
In the conventional technology whereby a bias voltage is controlled in accordance with various kinds of information obtained from the plasma, etching selectivity changes due to a variation in bias voltage so that this technology is not suitable for a mask and a sample having an underlying film with a small thickness in some cases.
In addition, the conventional technology whereby the voltage of the high-frequency bias is subjected to on-off control does not consider control to turn the voltage of the high-frequency bias on and off in accordance with process variations in the course of processing. Thus, much like the one described above, when the bias voltage (that is, the on-off voltage value Vpp) is controlled in accordance with various kinds of information extracted from the plasma, the effect on the select ratio decreases in comparison with the continuous bias. For the select ratio with respect to a thin underlying film used in a device miniaturized to a level not exceeding 0.1 microns, however, the reduction of the effect on the select ratio still cannot be said to be sufficient.
It is thus an object of the present invention to provide a plasma processing method and a plasma processing apparatus that are capable of fabricating a device with a fabricated-line dimension of up to 1 micron while suppressing variations in fabricated-line dimension from wafer to wafer with a high degree of reproducibility without decreasing the throughput.
In order to achieve the above object, there is provided a method for carrying out plasma processing on a substrate by controlling the application of a bias to the substrate independently of generation of plasma, whereby the output value (the amplitude) of a high-frequency voltage applied to a sample base is subjected to periodical time modulation and a duty ratio of the periodical time modulation is changed for each processed substrate or for each plurality of processed substrates where the duty ratio is defined as a ratio of a sub-period, during which a large voltage is applied, to a period of the periodical time modulation.
In addition, in order to achieve the above object, a plasma processing apparatus for processing a substrate placed on a sample base installed in a vacuum container, in which plasma is generated and a high-frequency voltage is applied to the sample base, is provided with:
a high-frequency power supply connected to the sample base;
a modulation unit for periodically carrying out on-off modulation on the high-frequency voltage generated by the high-frequency power supply; and
a control unit for changing a duty ratio of the on-off modulation for each processed substrate or each plurality of processed substrates.
Furthermore, a width of a line obtained as a result of fabrication of a wafer is measured and the duty ratio is changed in such a direction that the measured line width""s shift from a prescribed value is corrected in case such a shift exists. As an alternative, the apparatus"" state having a correlation with a fabricated-line dimension is monitored and the duty ratio is changed so that the monitored quantity is restored to a normal range in case the monitored quantity is shifted from a normal value. An example of the apparatus"" state having a correlation with a fabricated-line dimension is a plasma emitted light.
It is to be noted that, in a method whereby a variation in apparatus state is monitored and fed back to correct etching conditions in order to stabilize a specific etching characteristic such as a fabricated-line dimension, it is necessary to prevent an etching characteristic other than the specific etching characteristic from changing because a certain condition is altered. An example of the other etching characteristic is the etching-speed wafer-surface uniformity. In accordance with the present invention, the output value (the amplitude) of a high-frequency voltage applied to a sample is subjected to time modulation and the duty ratio of the time modulation is changed to vary only the number of radiated ions and a radical sticking amount so that variations in fabricated-line dimension can be suppressed without affecting other characteristics such as the plasma composition and the plasma distribution.